1. Field of the Invention
The invention pertains to the art of electrolytic accelerometers used as tilt sensors and, more particularly, to devices of this type which include means for providing compensation for acceleration errors when used in a moving vehicle.
2. Description of the Prior Art
The invention relates to apparatus for sensing and measuring changes in inclination. Such apparatus has many applications; for example, in one form the apparatus may be used to sense or measure the variations in tilt of a surface from a datum position such as the horizontal. Thus, the invention may be used to measure accurately the grade of a road bed being traversed. Heavily loaded trucks may be unable to negotiate a road beyond a predetermined grade, and hence would find useful a device capable of measuring the grade while in motion and before stopping. Highway departments would find such a device useful for measuring the grades of highway under construction or for surveying roads about to be modernized.
Because a moving vehicle subjects the sensing apparatus to longitudinal accelerations when it accelerates or decelerates in a forward direction, it has been necessary to compensate for the effects of such acceleration in order to obtain an accurate reading of road grade. However, such apparatus used heretofore has been characterized by an inability of the gravity reference sensor to distinguish between actual tilt of the vehicle from a horizontal reference axis and the influence thereon of the horizontal components of acceleration. Because of the frequent accelerations encountered by the vehicle and therefore the sensing equipment, expensive and complex gyroscopic equipment has previously been used for the measurement of instantaneous grades of road beds. This may include a conventional gyroscopic apparatus such as that described by Shigeo Kubo in U.S. Pat. No. 4,219,940, "Angular Accelerometer Stabilized Pendulum", which provided a pendulum capable of maintaining its alignment with the local gravity vector independently of horizontal acceleration. Another accelerometer for use in vertical angle measurements is described by Klas R. Wicklund in U.S. Pat. No. 4,277,895, "Apparatus for Indicating Angular Position in a Vertical Direction" which provided a pendulum driving a coil rotatable in a magnetic field capable of supplying a current output indicative of the degree of rotation of the pendulum with respect to a vertical reference position. However, these devices are complex mechanically, expensive, and lack the robustness required for vehicular service. Another accelerometer of the liquid level type was described by C. G. Buckley, G. A. Bhat and Harold L. Swartz in U.S. Pat. No. 4,028,815, assigned to the assignee of the present invention. While providing a relatively simple and rugged sensor capable of measuring angular displacement, this level is incapable of distinguishing the angular displacement from the longitudinal accelerations.
The present invention provides a gauge for compensating the acceleration sensitive response of a liquid level sensor gravity reference by utilizing an independent sensing of velocity and processing the value obtained to compensate the output of the sensor to yield a measurement of road grade free of acceleration effects. Beneficially, this approach eliminates mechanical gears and linkages and substantially reduces the cost, while providing improved effectiveness and accuracy.